Steel Metal

ackground

Worldwide, HALDENWANGER is the only manufacturer of rollers for kilns and furnaces able to supply the complete programme of roller materials. With our know-how of producing different mullite and SiC-based materials we offer custom made solutions.

AZoM - Metals, Ceramics, Polymer and Composites : Haldenwanger ceramic rollers for furnace applications
Roller Hearth Kilns and Fast Firing

Roller hearth kilns for fast firing of wall and floor tiles have been used for nearly 25 years. In this type of kiln the products to be fired are transported through the kiln by means of rotating ceramic rollers. Since the beginning HALDENWANGER has supplied the various manufacturers and users of kilns with oxide and non-oxide ceramic rollers. In the past, the diameter of rollers was up to 55 mm with a length of approx. 2000 mm. Because of increased capacity demands, the wall and floor tile industry asked for more efficient kilns. That is the reason why kilns became wider and therefore the rollers became longer. Today there are roller diameters of 30, 33.7, 40, 42, 45 and 50 mm for maximum roller length of approx. 3700 mm. This corresponds to a useable kiln width of approx. 2500 mm.
Roller Tolerances

Roller tolerances have also become much tighter. Today, diameter tolerances of +/- 0,2 to +/- 0,3 mm are typical and roller bending is as small as 1,0 to 1,5 mm which corresponds to a TIR of 2-3 mm. Additionally, the overall length taper specification of no more than 1mm is today a standard requirement.
Production Capabilities

Production capacity allows us to produce nearly all standard dimensions. The rollers must have good refractory properties to guarantee a perfect operation for a given temperature and load. Furthermore, there is the need to change rollers during kiln operation. In such cases, the high thermal gradients which rollers have to withstand means that a material must be selected having sufficient thermal shock resistance.
Ceramic Roller Materials and Their Applications

Oxide ceramic rollers, these are our materials Sillimantin 60, Sillimantin 65, Sillimantin 60 NG, are used in roller hearth kilns for firing wall and floor tiles. Non-oxide ceramic rollers, these are our materials Halsic-R (RSiC) and Halsic-I (SiSiC), are used in roller hearth kilns for firing sanitary ware and porcelain. These materials are used because of the better creep resistance compared to the oxide ceramic rollers.
Installation of Rollers

Before installing the rollers in the kiln they must be completely dry. This is very important if the rollers are changed during operation. Drying is usually done by storing the rollers on or above the furnaces (utilisation of waste heat).
Reducing Heat Losses by Plugging Open Ends

Heat losses in the kiln arise from the open ends of the roller. By using ceramic fibre plugs the rollers are sealed at both ends. The plugs usually have a maximum length of 100 mm to 200 mm.
Avoiding Bending of Rollers

The oxide ceramic rollers must continue to rotate whilst they are heated up or cooled down. This is very important, for example, if a roller hearth kiln is to be shut down for maintenance or technical reasons.

Stationary oxide ceramic ollers, even for a short time, results in roller bending. The drive to the rollers should never be switched off even at kiln temperatures of 500 °C to 600 °C, as frequently occurs. Stationary rollers subjected to the above-mentioned temperature will be damaged! When the kiln is re-started, considerable breakage is to be expected. The rollers must also be kept rotating even over short breakdown periods.
Application of Haldenwanger Ceramic Rollers

HALDENWANGER ceramic rollers are used for the firing of:

·         Wall and floor tiles up to 1300 °C

·         Tableware porcelain up to 1420 °C

·         Sanitary porcelain up to 1300 °C

·         Ferrites up to 1300 °C on supporting setters

·         Heat treatments of metals up to 1200 °C.
Ceramic Supporting Rollers for Glass Tempering Furnaces

In glass tempering furnaces the glass is transported on rollers through the furnace. Product is run directly on the rollers, i.e. without supports.

The tempering time can be regulated via the rotational speed of the rollers as well as by applying different heating systems.

All large sized glass plates are subject to this secondary thermal treatment because of new regulations. (single pane safety glass)
Reasons for Tempering Glass

The aim of this process is to cause pre-stressing the glass through heat, so that there are no sharp splinters if the glass breaks. Like safety glass, the pane must break into many harmless particles.
Properties of Ceramic Rollers for Glass Tempering

Because of the sensitivity of the glass surface, the ceramic rollers used must be very smooth and free of the smallest undulation. The bending and the concentricity are very important factors, too.

Furthermore, the ceramic material must have good thermal shock resistance properties in order to withstand changes of temperature occurring in the process.
Materials for Glass Tempering Furnace Rollers

The material used for these rollers is our Fused Silica. In use today are solid as well as hollow fused silica rollers which can be supplied with mounted metal end caps.

Background

The 1980s was a boom time for engineering ceramics. Emerging materials such as silicon nitride and SiAlONs were seen as the shape of things to come in several industries, replacing metallic components on a grand scale. For more than a decade, intensive effort went into these new materials. One goal was to produce an all ceramic car engine. However, such an all ceramic future never materialised as reality failed to match up to expectations. Today, ceramics have an important part to play in conjunction with other materials. They can add particular functionality or provide added benefit to a component, eg as hard wearing surfaces, ultra-hard materials in cutting tools, for corrosion resistance or high temperature protection. As ceramics are not being used in isolation, joining is an increasingly vital technology for the integration of the materials.
Joining of Ceramics

Despite its obvious importance, joining is often neglected during the design process. Many engineers incorporate ceramics into a component as though they were high performance metals, giving little thought to service conditions or joining operations. This can lead to two outcomes, either the part fails and its designers conclude that the ceramic was unsuitable and that metals should be used as before, or an expensive redesign may be required if a ceramic must be used.
Joining Considerations

There are many important issues to be considered alongside joining such as materials selection, best practice and joint design, but this article concentrates on ceramic joining technologies and, in particular, some of the novel ways of producing ceramic/ceramic and ceramic/metal bonds.
Ceramic Joining Technologies

The ceramic joining technologies used today (few of which have been developed specifically for this class of materials) range from simple mechanical attachment such as the compression fit used in spark plugs, figure 1a, through to liquid phase processes such as adhesive bonding and brazing. The thermal protection system for the space shuttle uses adhesives, and the ceramic turbocharger rotor uses brazing, figure 1b. There are problems associated with these processes including processing considerations (such as component size and joining atmosphere), time constraints and costs, which is why the new ceramic joining technologies described in this article are being developed.

Figure 1. (a) Spark plugs manufactured using compression fitting and (b) turbocharger rotor assemblies made from silicon nitride
Ultrasonic Joining

Ultrasonic joining, which is used extensively in the plastics industry, has been used for ceramic/metal combinations such as alumina/aluminium, alumina/stainless steel, zirconia/steel and glass ceramic (cordierite-based)/copper. Typical applications include batteries, thread guides, textile cutting equipment and heavy duty electrical fuses. The advantages of the process are the very fast joining times (less than one second), the fact that surface preparation is not critical (contrary to almost every other ceramic joining process) and the lack of melting and intermetallic formation. However, to join hard metals such as steel, soft, deformable interlayers are needed. One limitation of ultrasonic joining of ceramics is that only films or thin sheets of metal can be joined to a ceramic.
The Process

Ultrasonic joining requires a transducer assembly operating at about 20kHz (the source of the ultrasound) coupled to a sonotrode. The sonotrode tip is placed in contact, usually under a clamping load of 1-10Nmm-2, with the workpiece, figure 2. The heat generated is localised at the interface, creating a temperature of up to 600°C when using aluminium interlayers.

Figure 2. An ultrasonic welding set up.

The bonding mechanism relies on the vibratory shear stress of the metal exceeding its elastic limit, coupled with the breakdown of surface oxide films exposing atomically clean metal. The clamping force exerts plastic deformation on the metal, which increases the interfacial contact between the metal and ceramic. Mechanical keying then occurs across the interface and the joint is formed, perhaps along with some chemical interactions.
Applications

In future, ultrasonic joining could be applied to gas-tight container seals (lamps), optical components and joining metallic membranes onto ceramic bodies.
Transient Liquid Phase Bonding

Another technique, transient liquid phase bonding (TLPB), has the ability to produce a bond at a lower temperature than that at which it will be ultimately used. The technology is currently being adapted for a number of ceramics using ‘interlayers’ based either on glasses (such as oxynitrides for joining SiAION) or pure metals or alloys (such as Ge and Ge-Si for joining SiC and SiC/SiC composites).

Bonding in the SiAlON system is shown in figure 3. A mixture of silicon nitride, yttria, silica and alumina are applied by spray coating to one surface of the joint. As the samples are heated to 1600°C, a load of 2MNm-2 is applied. Joint formation occurs at this temperature over a period of 10-80 minutes. At about 1400°C, the oxide components react to form a Y-Si-Al-O liquid phase. This leads to densification and sintering. The silicon nitride then dissolves into the liquid, boosting both Si and N contents and altering the composition to Y-Si-Al-O-N. At the same time, (ß-SiAlON grains grow and form an interlocking network across the joint, forming an in-situ reinforcement phase. A secondary process is the diffusion of the adhesive mixture into the surrounding adherent material. Within this diffusion zone the composition and properties of the ceramic gradually change.

Figure 3. The transient liquid phase bonding process.

The weakness of the TLPB method is that a favourable reaction between the interlayer and the substrates is required. For silicon nitrides, the glassy interlayer must redistribute itself and penetrate the adjoining microstructure.
Infiltration Processes

High strength, high temperature materials for operation in excess of 1000°C, such as SiC-based composites, are needed for structural applications such as heat exchangers and gas turbine components. This is because traditional stainless steels and superalloys have reached their operational limits. Industry therefore wants to develop a robust joining process suitable for both SiC monolithic and composite materials. Polymers that react with and infiltrate into the bulk material offer a potential solution.

In these so-called infiltration processes, a mixture of polymer precursor (a source of carbon), aluminium, boron and silicon is applied to the joint surfaces (in a tape, paste or slurry) and then heated, generally to 1200°C, in an inert atmosphere and/or air using a propane torch or furnace. The joint forms through pyrolysis of the polymer precursor material, which subsequently reacts with the silicon in the presence of aluminium and boron sintering aids to form in-situ, high density SiC. Strengths range from about 95 MNm-2 for air-joined samples down to 40 MNm-2 for argon atmosphere samples. The fact that the joint can be produced using a simple gas torch could have a major impact on the repair or on-site production of ceramic/ceramic joints. So far, various grades of SiC, SiC/SiC and C/SiC composites have been joined using this technique.
Microwave Joining

Microwaves provide another technique for joining ceramics together. Microwave energy is already being applied to the drying/firing of refractories and whiteware. Now it is being considered as an energy source for joining ceramics such as alumina, zirconia, mullite, silicon nitride and silicon carbide. The direct coupling of the microwave with the ceramic results in volumetric heating, and so there is great potential for heating large sections uniformly. Control of the location of the maximum electric and magnetic fields also enables precise, selective heating.

Conventional diffusion bonding techniques use radiant heating methods and so the time to reach temperature and the time at bonding temperature can be as long as 8 hours. This is particularly the case for materials such as alumina, which are diffusion bonded at temperatures approaching 1600°C. Using a microwave heat source, bonding times can be reduced by an order of magnitude.

Very high purity aluminas are difficult to heat, owing to low inherent dielectric properties making joining difficult. Impure, 85% alumina on the other hand is joined easily. The use of interlayers, including sealing glasses, and alumina gels have both been investigated for producing joints with high purity alumina. Alumina gels offer the advantage that, at the joining temperature, the gel transforms into colloidal a-alumina which subsequently sinters to provide an homogeneous interface. Joints between 85% alumina show bond strengths equivalent to that of the parent material. Joint formation has been studied and a number of possible mechanisms have been identified, depending on the material. For impure aluminas, the glassy grain boundary phase softens and assists in the bonding process, while for zirconia, a solid state process has been identified.
Brazing

Work at TWI is focused mainly on the more traditional methods of joining ceramics, such as brazing, diffusion bonding, glasses and adhesives. However, development programmes are also investigating the modification of braze alloys by the addition of a ceramic reinforcement. This reinforcement provides a joining medium with a coefficient of thermal expansion (CTE) between that of ceramics and metals, and also gives the joint improved strength owing to the introduction of a second phase ie an in situ metal matrix composite. The ability to tailor the CTE of the joining medium is of greater interest. The additional joint strength is a bonus as this raises the possibility of designing the braze to accommodate thermal stresses that would otherwise build up during the joining process.
Sol-Gel

For low temperature applications, novel organic based adhesives reinforced with nano-sized ceramic particles are being fabricated at TWI using sol-gel chemistry, a liquid phase process. This technique allows intimate mixing of the ceramic and organic constituents on a molecular scale, producing materials of high purity and with a high level homogeneity. Sol-gel processing provide a joining medium that is tough yet flexible and can be designed to be hydrophobic and self repairing. Applications for these strong, modified adhesive may be found in the optical, biomedical an defence industries.
Summary

Looking at the field of ceramic joining processes as a whole, techniques have evolved from derivations of metal and plastic joining methods into discrete technologies. However, improvements and modifications to these existing technologies are still required to make them more readily adaptable. In association with this, the selection of the appropriate material and joint design are critical facto when developing joining technologies.

More and more developments in ceramic joining are beginning with a design exercise involving the modelling of new component designs and joint geometries. Modelling is very significant for introducing ceramics into new applications, as it can lessen the number of expensive trial and error failures in material selection, joint design and the selection of the appropriate joining procedure and process cycle.
Modelling

The difficulty faced in the modelling approach is the lack of reproducible material property data for ceramics and joining media such as brazes, solder adhesives etc. If incorrect data is put in, incorrect joint designs will be produced and will give poor support to the decision making process. A great deal of work is therefore concentrating on the development of appropriate data to plug into the models. Until then, the use of modelling for ceramics joint development will continue to be backed up by a series of practical trials.

Background

Ceramics, and in particular, engineering ceramics, are becoming increasingly important in today’s society. They offer a huge array of mechanical, thermal and electrical properties. These range from low thermal conduction in ceramics such as alumina, to high thermal conduction, e.g. diamond, and from low density electrical insulators to superconducting ceramics. Piezo-ceramics offer the almost unique ability to convert electrical energy into mechanical movement and vice versa, and materials such as beta-alumina and zirconia, which exhibit ionic conduction, are used as sensors. These materials have found application in a wide range of industries, but their exploitation has been concentrated mainly in the automotive, power generation and electronics sectors.
The Advanced Ceramics Market in the USA

A recent study of the advanced ceramic market in the USA shows that the use of advanced ceramic materials is increasing, and will continue to increase. The dominant market sector is electronics, with 65% of the market. This segment, which contains mature technologies such as integrated circuits and capacitors, alongside relatively new areas such as piezo-ceramics and superconductivity, is forecast to continue to grow steadily well into the next century. The second highest growth rate is in structural ceramics, where materials such as silicon carbide, silicon nitride, zirconia, diamond and ceramic matrix composites are being exploited as cutting tools, wear parts, sensors and biomaterials.
Ceramics in Application

It should be remembered that, in general, ceramics should be used only where their specific properties are required. A common thread amongst all of the above applications is that, at some point, the ceramic component interfaces with something else - another part of the machine/device made from another material, or perhaps a casing. At this point some kind of attachment or joint is required.
Ceramic Joining Techniques

Ceramics are inherently difficult to join either to themselves or metal structures - a consequence of their strong ionic and covalent bonding. However, there are several well-established technologies available, including mechanical attachment, adhesives, soldering/brazing and glass-metal sealing. These are often chosen on the basis of temperature requirement of the joint, ease of implementation, functionality etc. There are also other more unusual or application-specific processes, such as microwave bonding, ultrasonic welding and friction welding. Of the many joining processes available, probably the main and most adaptable technique used to join ceramics is brazing.
Ceramic Brazing

Brazing is a liquid phase process that is particularly well suited to preparing joints and seals, and is an established technique for the joining of ceramics. The brazing process can be readily adapted to the mass production of components, such as those used in the electronics and automotive industries.
Brazing

The dictionary definition of brazing is stated a ‘the joining of two pieces of metal by fusing layer of brass or spelter between the adjoining surfaces’, and is probably a derivation of a 16th century French word meaning ‘to burn’. The process basically involves a braze melting and flowing between the two pieces of material. This is commonly referred to as ‘wetting’ and is absolutely critical - particularly when brazing ceramics. Nowadays there are many materials that can be fused to produce joints between materials - those that melt above ~450 °C are classed a, brazes, materials melting below ~450 °C are called solders.
Wetting

Worldwide Leaders in Technical Ceramics

A recognized worldwide leader in the engineering and manufacturing of technical ceramics, CoorsTek provides advanced materials and design configurations for ballistic-grade ceramics used in appliqué and integrated armor systems.
Performance Advantages

CoorsTek CeraShieldTM ceramics offer many advantages over conventional materials. Our specially formulated ceramic armor materials provide:

·         Low weight

·         High hardness

·         Controlled, uniform microstructure

·         Dimensional stability over wide temperature range

·         Compatibility for use in finished armor systems with proven ballistic performance for National Institute of Justice (N.I.J.) and international design standards(C).
Variety of Materials for a Variety of Armor Applications

CoorsTek offers a wide range of CeraShieldTM materials to satisfy your specific ballistic application:

·         High-density aluminum oxides

·         Zirconia-toughened aluminas

·         Boron carbides

·         Silicon carbides

Applications of CoorsTek Ceramic Armor
Advanced Ceramic Vehicle Tiles

Exceptionally well-suited for vehicular applications, CoorsTek® produces lightweight technical ceramic tiles using tight-tolerance manufacturing standards, high-purity formulations, and controlled microstructures to ensure durable, everyday performance and a consistently reliable defense to unexpected threats.

·         Shapes and Sizes for All Needs

CoorsTek offers CeraShieldTM advanced ceramic vehicle tiles in a variety of shapes and sizes. Choose from square or hexagon shapes at various imperial and metric measurements, or let our specialists help you select custom shapes, thicknesses, and configurations to optimize your armor system design.

·         Modular Assembly

CoorsTek provides assembly services to form multiple tiles into custom panels of complex shapes and configurations – including holes & cutouts. Perfect for aircraft, armored vehicles, and executive vehicles, our modular panels provide a custom fit and superior performance.

·         Vacuum-Bonding Capabilities

We’ve perfected the bonding process to perform under some of the harshest industrial environments on the planet. Our specialized process bonds the ceramic to a variety of customer specified substrate materials for a thoroughly robust and integrated solution.

AZoM - Metals, ceramics, polymers and composites - CoorsTek ceramic armor components for vehicular armor applications.

Figure 1. CoorsTek ceramic armor components for vehicular armor applications.
Next-Generation Ceramic Torso Plates

CoorsTek offers lightweight, superior-quality torso plates in a wide variety of shapes, sizes, thicknesses, and materials including:

·         CeraShieldTM High-Density Aluminum Oxides

o        The traditional monolithic plate material, CoorsTek offers extremely high-volume, multi-facility production capacity

o        Standard sizes and shapes including all SAPI sizes.

o        Custom thicknesses available from 1 mm and up

·         CeraShieldTM Silicon Carbides

o        Superior hardness for advanced threats

o        Lower weight with higher performance benefits

·         CeraShieldTM Boron Carbides

o        Lowest density for ultra-lightweight designs

Representative examples of our standard plates are illustrated below. Most sizes and shapes are offered in all CeraShieldTM materials. Custom shapes, sizes, and thicknesses are readily manufactured to customer specification.

Abstract

To fabricate a SiC-on-insulator (SiCOI) structure applicable to pressure sensors, the growth of cubic silicon carbide (3C-SiC) crystalline films on thermally oxidized Si (SiO2/Si) substrates by triode plasma chemical vapor deposition (CVD) utilizing hydrogen radicals generated in the H2 plasma and monomethylsilane (MMS) was investigated. Piezoresistive property of the 3C-SiC films grown was investigated. Under negative grid bias conditions, the (110) oriented crystal films were grown at a substrate temperature of 600°C. From the variation in the resistance of a SiC (110) oriented film grown on SiO2 layer, gauge factor was estimated to be about -2.2.
Keywords

piezoresistive property, silicon carbide, triode plasma CVD, monomethylsilane, gauge factor.
Introduction

Cubic-silicon carbide (3C-SiC) is of great interest for high temperature and high power applications due to its outstanding properties, including chemical inertness, thermal stability, and high saturated electron drift velocity. Its high stiffness, high mechanical strength and extreme chemical inertness also make it suitable for the development of electronic devices such as pressure sensors and for use as micro-electromechanical system (MEMS) in chemically and physically harsh environments at high temperatures [1, 2]. For some applications to the pressure sensor and the MEMS, SiC films are required to be deposited on insulators such as SiO2, the so-called SiC-on-insulator (SiCOI) structure, because the active layer must be electrically isolated towards the substrate.

Several methods of fabricating the SiCOI structure have been reported, namely, 1) the growth of cubic SiC (3C-SiC) on a Si-on-insulator (SOI) substrate [3, 4], 2) ion implantation and wafer bonding [5], and 3) the direct growth of 3C-SiC on an insulator [6, 7]. Among these methods, the direct growth of 3C-SiC on an insulator is the most desirable method because it is the simplest and has a good cost performance. For the application of SiC films to the pressure sensors, (100) or (110) oriented crystalline films are required due to their large gauge factors. In the case of the epitaxial n-SiC films, large gauge factors of –31.8 by the application of a tensile stress in the <100> direction and of –3.7 by the application of a tensile stress in the <110> direction have been obtained [8]. These dependences originated from the conduction band structure. However, when SiC is grown on the insulator such as SiO2 or Si3N4 by low-pressure CVD (LPCVD) and sputtering, polycrystalline films are formed[6, 7].

To date, we have investigated the epitaxial growth of 3C-SiC on Si substrates by triode plasma CVD [9], which utilizes the high-density hydrogen radicals for the film growth at low temperatures. By the triode plasma CVD using monomethylsilane (MMS) as a source gas, the heteroepitaxial growth of 3C-SiC on Si (100) was achieved above 900°C[9]. From the results, it is considered that crystal growth would be greatly enhanced by the reaction of high-density hydrogen radicals with precursors from MMS even at low temperatures. If high-quality (100) or (110) oriented SiC films were grown directly on SiO2, SiC pressure sensors could be fabricated by a simple method.

In this study, we investigated SiC crystalline film growth on SiO2/Si substrates by triode plasma CVD using MMS and dimethylsilane (DMS) as source gases to fabricate SiCOI structure and evaluated the piezoresistive property of the SiC film.
Experimental Procedure

Figure 1 shows the schematic diagram of the triode plasma CVD apparatus used in the SiC growth. The triode plasma CVD system has a wire mesh electrode (grid, diameter = 135 mm) placed between the cathode (diameter = 85 mm) and the anode (diameter = 100 mm) of a conventional diode type rf plasma CVD chamber. The cathode is surrounded by a grounded cylindrical stainless steel wall (diameter 135 mm). The grid (wire diameter 0.3 mm, wire spacing 1.3 mm) is connected to the grounded wall. By applying various dc biases on the grid (100 ~ -100 V), we attempted to control the impingement of the charged particles on the substrate surface. And the low electron temperature (<1 eV) in the afterglow region between grid and anode was found to be obtained under the negative biases (-50 ~ -100 V) using double probe method, as shown in Figure 2.

AZoJomo- AZo Journal of Materials Online - Schematic diagram of triode plasma CVD apparatus.

Figure 1. Schematic diagram of triode plasma CVD apparatus.

AZoJomo- AZo Journal of Materials Online - Variation in the electron temperature and electron density in the afterglow region measured by a double probe.

Figure 2. Variation in the electron temperature and electron density in the afterglow region measured by a double probe.

Substrates were placed on an electrically floated anode. Base pressure was below 6.7x10-5 Pa in the growth chamber, which was evacuated using a turbo-molecular pump and a mechanical rotary pump. Si (100) substrates, which were thermally oxidized in dry oxygen atmosphere at 1000°C for 6 hours in an electric furnace, were used. The thickness of the SiO2 layer was about 200-300 nm.

Experimental conditions are as follows: the distance between cathode and grid 20 mm, that between grid and substrate 20 mm, H2 flow rate 200 sccm, MMS and DMS gases (TRI Chem. Lab. Inc., 99.9999%) pressure during film growth 1.3x10-2 Pa, gas feed ratio H2/(MMS or DMS) = 85, substrate temperature 500-700°C, total gas pressure during probe measurement and film growth 133 Pa, rf power 100 W. Hydrogen plasma was generated by an rf (13.56MHz) discharge during the probe measurement without MMS.

The procedure of SiC growth is as follows. After evacuating the growth chamber below 10-4 Pa, the substrate temperature was raised to 300°C, which is lower than the decomposition temperature of the source gases, by a carbon heater. With the supply of H2 and MMS (or DMS), substrate temperature was rapidly raised from 300°C to growth temperature, leading to the film growth. At the same time, rf power was supplied to the cathode and the negative DC bias was added to the grid. The crystallinity, and Si-C bonding environment of SiC films were measured using an X-ray diffractometer (RIGAKU, RADIIIA; DS 1/2˚, RS 0.15mm, SS 1/2˚) and an infrared spectrophotometer (Hitachi Model 260-10).
Results and Discussion

Figure 3 shows the infrared (IR) transmission spectra of the SiO2/Si substrates before and after SiC growth at 600˚C for 3hrs. The spectrum of the substrate after SiC growth presents a sharp absorption peak at 795cm-1, which indicates the transverse optical (TO) phonon of stoichiometric SiC crystal [10]. The absorbance of Si-O bonds hardly changed after the film growth. In spite of the supply of high-density hydrogen radicals, oxide layer on Si substrate was hardly etched during the SiC growth by the triode plasma CVD.

Pure SiCTM CVD Silicon Carbide

CoorsTek manufactures bulk SiC using a high temperature Chemical Vapor Deposition (CVD) process.

Ultra-pure raw materials and carefully controlled processing conditions create exceptionally clean, dense, and corrosion resistant SiC. Pure SiC can be manufactured to meet both high and low resistivity requirements.

·         Purity greater than 99.9995%

·         Excellent mechanical properties

·         High thermal conductivity

·         Superior corrosion resistance

·         Near-net shape deposition

·         Sizes up to 20"
Reaction Bonded Silicon Carbide

CoorsTek employs a reaction-bonding process to manufacture SiC that retains approximately 10% free silicon.

Our bonded SiC can be formed by casting, dry pressing, or isostatic pressing.

·         Excellent wear properties

·         Thermal shock resistance

·         Sizes up to 20"
Direct Sintered Silicon Carbide

CoorsTek produces high-purity SiC using a direct sintering process. This process allows for low-cost forming methods such as casting, dry pressing, and isostatic pressing, while retaining high-purity levels.

·         Purity greater than 99%

·         Thermal shock resistance

·         Near-net shape forming

·         Excellent mechanical properties

·         Sizes up to 36" x 75"
Graphite Loaded Direct Sintered Silicon Carbide

CoorsTek has combined the lubricating properties of graphite into their already established high-purity sintered silicon carbide to create the next generation in friction and wear materials. This engineered material designated as SC-DSG offers the stability of sintered silicon carbide and the low frictional characteristics of graphite.

This chemically inert combination can be formed utilizing the latest technologies of dry pressing, casting, and isostatic pressing.

·         Excellent Thermal Shock Properties

·         Excellent Chemical Resistance

·         Near Net Shape Forming

·         "Built-in" lubrication

·         Sizes up to 20"
Materials and Manufacturing Experts

CoorsTek is uniquely capable of providing advanced materials and manufacturing technologies. Let the CoorsTek team help you select the best materials and design for manufacturability.

Background

In aero-engines, the blade of the high pressure turbine was for a long time the highest of the high technology in the aero gas turbine, and despite the complexity of the modern fan blade, the challenge it provides does not reduce. The ability to run at increasingly high gas temperatures has resulted from a combination of material improvements and the development of more sophisticated arrangements for internal and external cooling (figure 1).

Figure 1. Schematic of a gas turbine engine.
Modern Alloys

A modern turbine blade alloy is complex in that it contains up to ten significant alloying elements, but its microstructure is very simple. The structure is analogous to an `Inca wall’, which consisted of rectangular blocks of stone stacked in a regular array with narrow bands of cement to hold them together.

In the alloy case the `blocks’ are an intermetallic compound with the approximate composition Ni3(Al,Ta), whereas the `cement’ is a nickel solid solution containing chromium, tungsten and rhenium.
Superalloys

Superalloys have always contained phases of this type, but in recent years the titanium in the original intermetallic has been replaced by tantalum. This change gave improved high temperature strength, and also improved oxidation resistance. However, the biggest change has occurred in the nickel, where high levels of tungsten and rhenium are present. These elements are very effective in solution strengthening.

Since the 1950’s, the evolution from wrought to conventionally cast to directionally solidified to single crystal turbine blades has yielded a 250°C increase in allowable metal temperatures, and cooling developments have nearly doubled this in terms of turbine entry gas temperature. An important recent contribution has come from the alignment of the alloy grain in the single crystal blade, which has allowed the elastic properties of the material to be controlled more closely. These properties in turn control the natural vibration frequencies of the blade.

If metallurgical development can be exploited by reducing the cooling air quantity this is a potentially important performance enhancer, as for example, the Rolls-Royce Trent 800 engine uses 5% of compressor air to cool its row of high pressure turbine blades. The single crystal alloy, RR3000, is able to run about 35°C hotter than its predecessor. This may seem a small increase, but it has allowed the Trent intermediate pressure turbine blade to remain uncooled.
Continuing Developments

It is estimated that over the next twenty years a 200°C increase in turbine entry gas temperature will be required to meet the airlines’ demand for improved performance. Some of this increase will be made possible by the further adoption of thermal barrier coatings. These coatings are produced from ceramic pre-cursors and have the potential to contribute about 100°C through the protection they provide.
Thermal Barrier Coatings

Thermal barrier coatings have been used for some years on static parts, initially using magnesium zirconate but more recently yttria-stabilised zirconia. On rotating parts, the possibility of ceramic spalling is particularly dangerous, and strain‑tolerant coatings are employed with an effective bond coat system to ensure mechanical reliability.
Ceramic Matrix Composites

Further increases in temperature are likely to require the development of ceramic matrix composites. A number of simply shaped static components for military and civil applications are in the engine development phase and guide vanes have been manufactured to demonstrate process capability, such techniques involve advanced textile handling and chemical vapour infiltration.

However, it is the composite. ceramic rotor blade that provides the ultimate challenge. It will eventually appear because the rewards are so high, but it will take much longer to bring it to a satisfactory standard than was anticipated in the 1980’s. Research work has concentrated for some years on fibre reinforced ceramics for this application, as opposed to monolithic materials which possess adequate strength at high temperatures but the handicap of poor impact resistance.

Today’s commercially available ceramic composites employ silicon carbide fibres in a ceramic matrix such as silicon carbide or alumina. These materials are capable of uncooled operation at temperatures up to 1200°C, barely beyond the capability of the current best coated nickel alloy systems. Uncooled turbine applications will require an all oxide ceramic material system, to ensure the long term stability at the very highest temperatures in an oxidising atmosphere. An early example of such a system is alumina fibres in an alumina matrix. To realise the ultimate load carrying capabilities at high temperatures, single crystal oxide fibres may be used. Operating temperatures of 1400°C are thought possible with these systems.

Background

The rear end of the high-pressure compressor in an aero-engine is in a temperature environment set by the overall pressure ratio chosen for the engine cycle (figure 1). Since the 1950’s, this temperature level has risen by about 300°C. Titanium alloys have progressively improved in temperature capability up to 630°C, figure 2. This would allow most compressors to be designed completely in titanium. However, practice in the United States has been to switch at approximately 520°C to nickel alloys and incur a weight penalty.

Figure 1. Schematic of a gas turbine engine.

Figure 2. Progressive improvement of the temperature capability of titanium alloys has reached 630°C with IMI834

The development of IM1834 is a good example of the metallurgist’s response to the needs of the designer. The requirements were for higher tensile and fatigue strength and enhanced creep performance. These were met by optimising the structural balance between primary alpha content and the transformed beta phase in the titanium alloy.
Developments

Producing integrally bladed discs, or blisks, is a natural progression in that the blade attachment features are deleted, resulting in significant weight and cost savings. For small engines the most economic manufacturing method is to machine both disc and aerofoils from a single forging. There may be a penalty to pay in that the material strength of the aerofoil may be reduced compared to that of a forged blade. Attention to the forging method and to the manufacturing processes can overcome this.
Metal Matrix Composites

Titanium metal matrix composites can be applied to both aerofoils and discs. The use of silicon carbide fibre offers about 50% more strength and twice the stiffness of the high temperature titanium alloys, combined with reduced density. Aerofoil design will benefit from the increased stiffness due to selective reinforcement, providing the ability to control vibration modes and blade untwist. Further exploitation of this technique will be with integrally bladed rings which are expected to provide a 70% weight saving relative to a conventional geometry in titanium.
Intermetallics

Another material development project is the use of intermetallics. Compounds of nickel/aluminium and titanium/aluminium have been investigated with current emphasis on the latter. Most intermetallic compounds are brittle at room temperature. The first applications are therefore likely to be in small components such static and rotating compressor aerofoils where the advantages over titanium include higher specific strength and stiffness as well as improved temperature and fire resistance. The use of these materials could extend to more critical components. One possible application is as an alternative matrix to the titanium alloy in a metal matrix composite, although such an application will require alternative fibres, to minimise any thermal expansion mismatch, and novel processing technology.
The Future

Eventually, operating temperatures up to about 800°C will be possible, and intermetallics could offer a very attractive weight saving of around 50% compared with nickel-based alloys.

Blade Materials

Fan blades for high by pass aero-engines were, for many years, manufactured from solid titanium alloy forgings and were designed with mid span snubbers to control vibration. However, snubbers impeded airflow and reduced aerodynamic efficiency, penalising fuel consumption. Modern designs have deleted the snubber to provide a more aerodynamically efficient aerofoil, and increased the blade chord for mechanical stability, reducing the number of blades by approximately one third. This has been achieved at reduced weight with a hollow construction and an internal core.

For both snubbered and wide chord blades, a conventional fine grain titanium alloy - 6% aluminium and 4% vanadium (Ti6Al4V) is used. It is simple in terms of chemistry, with the aluminium offering strengthening and low density, and the vanadium making hot working of the material easier. It is used for discs and compressor blades up to about 350°C, but excellent superplastic forming and solid state diffusion bonding capabilities make it particularly suitable for the wide chord blade.

The low density core for the hollow design is an integral part of the structure. The two external skins are separated by either honeycomb filler or a superplastically formed corrugation which carries a share of the centrifugal load. Both panel-to-panel and core-to-panel joints must achieve parent material properties to withstand the effects of foreign body impact and fatigue.

For the first generation design the joints are made by a transient liquid phase diffusion bonding process, whereas the second generation employs solid state diffusion bonding in association with superplastic forming of the assembly. The cavity of the bonded construction is inflated at elevated temperatures between contoured metal dies using an inert gas to expand the core and simultaneously develop the blade’s external aerodynamic profile.

The reliability of these wide chord blades has been second to none. The step in technology produced a major competitive advantage and ten years passed before an equivalent design appeared from a manufacturer other than Rolls Royce. This service record was the result of thorough development testing. Fatigue testing in both low and high cycle modes was essential. Groups of blades were repeatedly accelerated to maximum speed in vacuum to establish low cycle endurance, and high cycle fatigue was investigated on a static vibration rig up to the maximum stress levels likely to be encountered in service.

With a large forward facing area, resistance to bird ingestion is required. Ingestion of a number of medium size birds has to be demonstrated by running an engine at take-off power and requiring it to ingest four birds within the space of one second. The engine continued to deliver power, accelerating and decelerating for a total period of thirty minutes to simulate the likely operating procedure following a severe ingestion incident.

In the very unlikely event of a blade mechanical failure, the engine has to be shown to be structurally sound and to contain all the debris, even if the failure occurs at maximum power. Containment in modern engines is achieved with aluminium or titanium casings through which the blade fragments can penetrate, to be caught in external windings of Kevlar.

As an indication of the benefits of materials development and design enhancements, engines incorporating the wide chord blade have fan modules that are approximately 24% lighter and an engine which is 7% lighter (typically the Trent 800 engine as used in the Boeing 777).

The Future

Looking to the future, some believe that carbon composite materials can be used to reduce weight. At present these materials limit the speeds for which the blade can be designed, requiring a greater diameter for a given thrust. It may be that this alternative approach will converge with the hollow design because airworthiness requirements have led to the incorporation of titanium sheathing around a large part of the composite blade with, of course, some weight penalty. The composite can be considered as an alternative core to the titanium honeycomb or corrugation. Rolls-Royce is studying the future possibility of titanium based metal matrix composites with selective reinforcement provided by silicon carbide monofilaments to control blade untwist.

Background

Titanium is a highly reactive metal. Fortunately the oxide film which forms spontaneously and is always present in oxidising environments is stable and self healing if damaged. It is this film which gives titanium its outstanding resistance to corrosion in a wide range of aggressive media.
Conditions for the Formation and Repair of the Titanium Oxide Film

The question often asked is, “what is meant by ‘oxidising environment’?” In fact the titanium oxide film is self supporting and self healing not only in oxidising environments, but also in neutral and mildly reducing conditions. For titanium ‘oxidising’ means conditions where a critical minimum level of oxidising agent is present in the media, under conditions where it can react with titanium preferentially to repair or inhibit further breakdown of the oxide film. This covers a wide range of chemicals, mixtures, process conditions and products.
Examples of Conditions for the Formation and Repair of the Titanium Oxide Film

In practice, air, moisture, moist gases, water, even heavily polluted brackish water and sea water will support the oxide film. Deaeration of an aqueous media does not affect the corrosion resistance of titanium, because water alone supports the oxide film. Only traces (e.g. ppm) of moisture or oxygen are required to maintain passive behaviour in most environments. Notable exceptions are chlorine gas, red fuming nitric acid and methanol where specific concentrations of moisture from 1.5% up to 10% are required to support the oxide film on titanium alloys under the worst conditions of environment and temperature.
Environments that will Attack Titanium

Pure reducing acids, hydrochloric, sulphuric, phosphoric etc, will attack titanium in proportion to concentration and temperature. Titanium is inhibited from attack, and its oxide film supported by the presence of limited concentrations of oxidising ions such as Fe3+, Cu2+ and others shown below. Inhibition becomes less effective as acid concentration and temperature increase. No inhibition is available to prevent breakdown of the titanium oxide film by hydrofluoric acid and acidic fluorides.

Moderate Inhibition
   

Strong Inhibition
   

Very Strong Inhibition

Oxygen, Ni2+, OCl-, picric acid, nitro nitroso and quinone organics.
   

ClO3-, cupric, ferric and mercuric ions, Ti4+, Te4+, Te5+, Se4+, Se6+.
   

Bromine, Chlorine, ClO4-, Cr6+, Mo6+, Mn6+, V5+, ions of Au, Ir, Pd, Pt, Ru
Enhancing the Corrosion Resistance of Titanium under Strongly Reducing Conditions

The corrosion resistance of titanium and its alloys in crevices or under deposits, (strongly reducing conditions and in the absence of oxygen or oxidisers) can be enhanced with palladium, ASTM Grades 7, 11, 16, 17, 18, 20, 24 or ruthenium, ASTM grades 26, 27, 28 and 29.